1. Field of the Invention
The present invention relates to a packet communication terminal, and more particularly to a packet communication terminal for sending and receiving a packet over a LAN (Local Area Network) such as Ethernet(R).
2. Description of the Background Art
As an access method used in Ethernet(R) and the like, CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is widely used. In CSMA/CD, upon sending a packet, a communication terminal detects a signal on a transmission medium and checks whether communication is currently being performed. If communication is not being performed, the communication terminal is allowed to send the packet. In the case where a plurality of communication terminals start sending packets substantially at the same time, packet collision occurs on the transmission medium. If a communication terminal detects a collision, the communication terminal stops sending a packet and retransmits the packet after a random time so as to prevent another collision.
Further, there exists a technique for sending a packet (emergency packet) which needs to be sent urgently over a LAN which performs communication using the above-described method. According to the technique, if there is a packet currently being sent on the transmission medium upon sending an emergency packet, a communication terminal to send the emergency packet deliberately causes a collision. The communication terminal then sends the emergency packet thereof while other terminals wait a random time for retransmission. This enables the communication terminal to send the emergency packet on a high-priority basis over the other communication terminals.
In the above-described CSMA/CD, however, while a communication terminal is sending a packet, other communication terminals cannot send packets until the sending of the packet has been completed. Therefore, in the case where communication terminals other than the communication terminal which is currently sending a packet need to immediately send packets (emergency packets), the communication terminals cannot send the emergency packets until the sending of the packet being performed by the communication terminal has been completed.
FIG. 13 is a diagram for describing a method of sending a packet in a conventional network system. The network system shown in FIG. 13 is configured such that communication terminals 91 to 94 are connected to a network 95. In FIG. 13, the situation, for example, where the communication terminal 91 is in the process of sending a packet to the communication terminal 93 is assumed. In this situation, even if a packet to be sent (the dashed-line arrow shown in FIG. 13) arises in the communication terminal 92, the communication terminal 92 cannot send the packet until the sending of the packet being in the process of communication (the solid-line arrow shown in FIG. 13) has been completed.
In addition, in the above-described technique where a collision is deliberately caused, the communication terminal only determines whether a packet to be sent therefrom is an emergency packet. That is, whether the packet on the transmission medium is an emergency packet is not considered. Thus, if the packet on the transmission medium is an emergency packet (from a viewpoint of other communication terminals), the emergency packet is broken down. This will be described using the example of FIG. 13. Assume that in the situation where the communication terminal 91 is in the process of sending a packet to the communication terminal 93, an emergency packet to be sent (the dashed-line arrow shown in FIG. 13) arises in the communication terminal 92. In this case, with the technique where a collision is deliberately caused, the emergency packet of the communication terminal 92 is always sent regardless of whether the packet currently being sent (the solid-line arrow shown in FIG. 13) is an emergency packet. Accordingly, in the conventional technique, there is a possibility that an emergency packet sent from a communication terminal may be broken down by another communication terminal, and thus an emergency packet cannot always be sent reliably.